The present invention relates to converting optical data signals to radio frequency data signals. More specifically, the present invention relates to converting broadband data transmitted on optical carriers in conventional wavelength-division-multiplexed (WDM) networks to corresponding microwave or millimeter wave carriers that support the continued broadband transmission of the data.
Wavelength Division Multiplexing (WDM) is a basic technology of networking optical signals. It is a technique by which a single fiber is used to carry many separate and independent optical channels. Each channel within the optical wavelength division multiplexed (OWDM) network is assigned a separate optical wavelength at which it is transmitted through the network. In a xe2x80x9csparsexe2x80x9d OWDM system, the optical wavelengths are (relatively) widely separated. For example, two optical wavelengths of 1300 nm and 1550 nm may be used in a sparse system. Such a system has an advantage of being easily implemented, but a major disadvantage is the limited number of optical channels that can be carried. In a xe2x80x9cdensexe2x80x9d OWDM system, the optical wavelengths are closely spaced. In a typical dense optical wavelength division multiplexed (DOWDM) system, the channel spacing may be as small as 1 nm or less. DOWDM systems provide substantially more channels than a sparse system, but are also more complex and difficult to implement.
OWDM technology provides the ability, in a given network, to allocate different services (or area of coverage) to different optical wavelengths for direct addressing. One example is in hybrid fiber coax WDM technology, where each service (broadcast video, pay per view, etc.) or different housing communities are routed by a designated wavelength in a Passive Optical Network (PON) architecture. Allocation of different services to different wavelengths simplifies the distribution of these services via optical networks, since the distribution hardware used in the networks does not need to know the type of service carried at each wavelength.
OWDM networks provide the capability to transmit large amounts of data between locations, but they have a fundamental limitation. OWDM networks require the use of optical fiber to move data from one point to another. Hence, OWDM networks may not serve areas where installation and maintenance of optical fiber is difficult and expensive. These areas may comprise rural areas where there are few users and these users are separated by significant distances, making the provision of fiber uneconomical. These areas may also include densely populated urban areas where the costs of interfering with the infrastructure and providing OWDM fiber to multiple locations may be prohibitively expensive.
OWDM networks may still be deployed in these areas, but they will generally be deployed in a relatively small area, servicing users who are closely located. Links to users on other networks may be accomplished by tying the networks together through the use of OWDM fiber or through the use of lower performing non-WDM data links. Coupling the separate OWDM networks through the use of OWDM fiber has the difficulty and expense factors discussed above. Non-WDM data links (such as radio frequency links or telephone lines) can be provided at a lower cost, but the networking capabilities inherent in a WDM network are lost.
A system for linking separate OWDM networks is described and claimed in the copending patent application entitled xe2x80x9cWireless Wavelength Division Multiplexed System,xe2x80x9d Ser. No. 09/897,747, filed on Jun. 29, 2001. The present patent application discloses the conversion of data transported on each optical wavelength in an OWDM system to a corresponding microwave or millimeter-wave frequency in a one-to-one correspondence. One technique which may be used for converting the several modulated optical carriers in an OWDM network to modulated radio frequency carriers transporting the same information as in the OWDM network, is to first de-multiplex the OWDM optical carriers and then detect the information in each channel using separate photodetectors. The photodetectors essentially convert the optical signals to electrical signals. The data in the individual electrical signals then modulates separately generated radio frequency carriers for wireless transmission with a one-to-one correspondence to the optical channels in the OWDM network.
However, the generation of low phase noise signals at microwave or millimeter wave frequencies using standard electrical frequency synthesizers may be a costly process, since several multiplication stages of a high quality, low frequency signal to the microwave or millimeter wave frequencies are required. Also, frequency synthesizers capable of this task can be quite bulky. Therefore, as the number of channels in the OWDM system, and hence in the converted wireless link, increases, the use of standard electrical frequency synthesizers to provide the required conversion can become detrimental in terms of cost and size.
Therefore, there exists a need in the art for apparatus and methods that provide for the conversion of the optical channels in OWDM system to radio frequency channels in a less costly and bulky fashion.
An object of the present invention is to provide a method and apparatus for converting optical channels in an OWDM network to wireless channels that may be radiated in free-space, in which the wireless channels have a one-to-one correspondence with the optical channels. It is a further object of the present invention to provide for conversion of the optical channels to wireless channels with a system that is small and inexpensive.
The present invention provides a method and apparatus for optical data channel to wireless data channel conversion. In addition, the present invention provides a method and apparatus for extracting optical carriers from optical data channels.
An optical to wireless converter according to the present invention, where the converter receives optical channels modulated at optical carrier frequencies and transmits corresponding wireless channels modulated at wireless carrier frequencies, comprises: a channelizer receiving the optical channels and producing extracted optical carrier signals; an optical frequency shifter receiving the extracted optical carrier signals and producing frequency-shifted extracted optical carrier signals; an optical heterodyne detector receiving the frequency-shifted extracted optical carrier signals and the optical channels and producing the corresponding wireless channels. The converter may additionally produce unmodulated wireless carrier signals. Preferably, the channelizer comprises an array of microsphere-based resonators in either a parallel or serial arrangement.
A method for converting optical data channels modulated at different optical carrier frequencies to wireless data channels modulated at different wireless carrier frequencies according to the present invention comprises the steps of: filtering the optical data channels to extract optical carrier frequency signals; frequency shifting the extracted optical carrier frequency signals by frequencies equal to corresponding wireless frequency carrier signals; and optically heterodyning the optical data channels and the frequency-shifted extracted optical carrier signals to generate wireless data channels modulated at the corresponding wireless carrier frequencies.
A converter according to the present invention for converting optical channels from an optical wavelength division multiplexed network to data modulated wireless channels in a one-to-one correspondence between the optical channels and the wireless channels, where the optical channels are combined in a composite optical signal, comprises: a wavelength division demultiplexer receiving the composite optical signal and producing demultiplexed optical channels; a channelizer receiving demultiplexed optical channels and producing extracted optical carrier signals, each extracted optical carrier corresponding to a different optical channel; a first optical combiner combining each extracted optical carrier signal, the first optical combiner forming a composite extracted optical carrier signal; an optical frequency converter receiving each extracted optical carrier signal, the optical frequency converter generating frequency-shifted extracted optical carrier signals, where the optical frequency converter shifts each extracted optical carrier signal by a different wireless carrier frequency to form the frequency-shifted extracted optical carrier signals; a second optical combiner receiving the frequency-shifted extracted optical carrier signals to form a composite frequency-shifted extracted optical carrier signal; an optical-heterodyne detector receiving the composite optical signal, the composite extracted optical carrier signal, and the composite frequency-shifted extracted optical carrier signal, the optical-heterodyne detector producing the data modulated wireless channels at the different wireless carrier frequencies, such that each wireless channel corresponds to a particular original optical channel.
An optical channelizer, according to the present invention, for filtering optical data signals to extract a plurality of unmodulated optical carriers comprises an array of microresonators, each microresonator of the array of microresonators comprising: a microsphere having a resonance; an input fiber disposed adjacent to the microsphere and coupling optical energy to the microsphere; and an output fiber disposed adjacent to the microsphere and coupling optical energy from the microsphere, where the resonance of the microsphere in each microresonator corresponds to an optical frequency of one unmodulated optical carrier of the plurality of unmodulated optical carriers.
An optical channelizer according to the present invention for filtering optical data signals to extract a plurality of unmodulated optical carriers, comprises: a fiber bus; and a plurality of microspheres, each microsphere being coupled to the fiber bus to receive optical energy, and each microsphere producing a corresponding unmodulated optical carrier
A method for filtering a composite optical signal containing a plurality of modulated optical data signals to extract a plurality of unmodulated optical carriers according to the present invention comprises the steps of: demultiplexing the composite optical signal to generate demultiplexed optical data signals; coupling each demultiplexed optical data signal to a corresponding microsphere in an array of microspheres; perturbing the corresponding microsphere to have resonance at or near an optical carrier frequency of the demultiplexed optical data signal coupled to the microsphere; and coupling the unmodulated optical carrier from the microsphere.
A method of filtering a composite optical signal containing a plurality of optical data signals to extract a plurality of extracted unmodulated optical carriers according to the present invention comprises the steps of: coupling the composite optical signal to a plurality of microspheres; perturbing each microsphere to have a different resonance, the resonance at or near an optical carrier frequency of a corresponding unmodulated optical; and coupling the corresponding unmodulated optical carrier from each microsphere to produce the plurality of extracted optical carriers.
An optical channelizer, according to the present invention, for filtering optical data signals modulating the optical carriers generated by a master mode-locked laser to extract unmodulated optical carriers comprises: an optical combiner combining the optical data signals into a composite optical signal; and a slave mode-locked and injection-locked laser receiving the composite optical signal, the slave laser being set to be below threshold and breaking into a mode-locked oscillation condition to generate an optical signal containing only the unmodulated optical carriers.
A method of filtering a composite optical signal containing a plurality of optical data signals modulating the optical carriers from a master mode-locked laser to extract unmodulated optical carriers according to the present invention comprises: coupling the composite optical signal to a slave mode-locked injection locked laser with a controllable cavity length; controlling the cavity length of the slave laser to cause it to break into a mode-locked oscillation condition, the oscillation condition creating an output signal containing only the unmodulated optical carriers; and coupling the optical output signal from the slave laser.